The first thing to do with this home computer before turning it on after many years that it was off is replacing all capacitors, which i did (some capacitors with acid leaks) and the computer worked well for about 15 minutes.

After the first 15 minutes while running a “for next” cycle, the computer started to show signs of slowing down and then freezed.

The problem was a RAM (4116) on BANK 2 [POSITION 416] (see photo), replaced the RAM the computer has started working again for another 15 minutes then it was freezed again.

I turned it off and on again and in this time a single line was repeated for X times, only with the composite video output, the VFD Display worked well (see photo)

Looking at the wiring diagram i found the area that manages the composite video output and i started to making measurements with the oscilloscope and after quite some time i have found the IC 74LS157 [POSITION 449] with a strange measurements that did not convince me for example the output pin 7 (OUTPUT Y2), in fact was broken, replaced the component the computer has started to work again.

Hopefully nothing will happen again: D

Gallery of the repair:

Useful link to visit: 8bit-homecomputermuseum.at